Fossil Primates, Living Apes, Man–Ape Comparative Anatomy and Erect Posture

1.5 Characteristics of Primates; Evolutionary trend and primate Taxonomy; Primate Adaptations (Arboreal and Terrestrial) Primate Taxonomy; Primate Behaviour; Tertiary and Quaternary fossil primates; Living Major Primates; Comparative Anatomy of Man and Apes; Skeletal changes due to erect posture and its implications

Official Syllabus: Paper I, Unit 1.5

This article covers Tertiary and Quaternary fossil primates, the living major primates, the comparative anatomy of man and apes and the skeletal changes due to erect posture. Primate characteristics, taxonomy, adaptations and behaviour are in Unit 1.5a: in brief, primates are defined by trends towards grasping hands, forward-facing eyes, reduced smell, larger brains and slow growth, and they are classified into strepsirhines (lemurs, lorises) and haplorhines (tarsiers, monkeys, apes, humans).

1. Tertiary and Quaternary Fossil Primates

In 1932, in the Siwalik Hills at Haritalyangar (Himachal Pradesh), the American geologist G. Edward Lewis found an ape upper jaw with small front teeth and named it Ramapithecus (1934). In the 1960s the American palaeontologists Elwyn Simons and David Pilbeam argued that its short face, small canines and thick enamel made it the earliest hominid, about 14 million years old, and textbooks placed India near the root of the human line. Then in 1980, on the Potwar Plateau of Pakistan, Pilbeam’s team found a partial face of Sivapithecus (specimen GSP 15000, about 8 million years old) that looked strikingly like an orangutan. Ramapithecus turned out to be the smaller (probably female) form of Sivapithecus, an ape on the orangutan line. One fossil face moved the human split from about 14 million to about 6–7 million years ago. The case shows why fossil primates matter: they test every claim about where we came from.

1.1 The time frame

The Tertiary period (Palaeocene to Pliocene, now divided into the Palaeogene and Neogene) and the Quaternary (Pleistocene and Holocene) together make up the Cenozoic era, the “Age of Mammals”. The dates below follow the current international time scale; older textbooks give rounder or earlier figures.

Period	Epoch	Began (million years ago)	Primate event
Tertiary (Palaeogene)	Palaeocene	66.0	Primate-like mammals (plesiadapiforms)
	Eocene	56.0	First true primates: adapids and omomyids
	Oligocene	33.9	First anthropoids (Fayum); monkeys reach South America
Tertiary (Neogene)	Miocene	23.0	“Planet of the apes“: hominoid radiation
	Pliocene	5.3	Early hominins; modern monkey groups
Quaternary	Pleistocene	2.58	Homo; giant apes and monkeys; ice ages
	Holocene	0.0117	Modern primates; extinctions caused by humans

Older books give Palaeocene 65, Eocene 53, Oligocene 37, Miocene 22.5 and Pleistocene 1.8 million years; the Pleistocene base moved to 2.58 million years in 2009.

Why geography matters. The old supercontinent had split into Laurasia (north) and Gondwanaland (south) in the Mesozoic. Until about 45 million years ago North America and Europe were linked through Greenland, so the same early primates occur on both. Africa was an island continent until about 16 million years ago, when the Arabian plate joined it to Eurasia and African apes spread into Europe and Asia. Cooling and drying through the Cenozoic shrank the forests and opened woodland and savanna.

1.2 Palaeocene: primate-like mammals

• Purgatorius (about 66 million years ago, at the very end of the Cretaceous, Montana, USA) is known from teeth and ankle bones. Its molars had the rounded cusps of a fruit- and insect-eater, and its ankle allowed the rotation used in climbing. It still had the primitive dental formula 3.1.4.3.
• Plesiadapiforms, such as Plesiadapis of North America and Europe, were squirrel-sized to cat-sized. They had claws, no postorbital bar, eyes facing partly sideways, and large rodent-like front teeth with a gap behind them.
• Status: most experts treat them as archaic relatives of primates rather than true primates, because their front teeth are too specialised to be ancestral. Carpolestes (about 56 million years ago), with a grasping big toe bearing a nail but sideways eyes, shows that grasping evolved before stereoscopic vision (Unit 1.5a §4.1).

1.3 Eocene: the first true primates

The Eocene was the warmest epoch of the Cenozoic, with forests reaching the poles, and it was the heyday of the prosimians: almost four times as many genera are known from the Eocene as live today. True primates (euprimates) with nails, forward eyes and a postorbital bar appear suddenly at its start. They fall into two families.

Family	Like which living primates	Traits	Examples and sites
Adapidae (adapoids)	Lemurs	Larger, active by day, leaf- and fruit-eaters, long snout	Notharctus, Smilodectes (North America); Adapis (France, described by Cuvier in 1821); Cantius
Omomyidae (omomyids)	Tarsiers	Small, large eye sockets (night-active), insect-eaters, leaping	Teilhardina (Asia, Europe, North America); Necrolemur (Europe)

Both families occur in India: the Vastan lignite mine near Surat, Gujarat, at least 53 million years old, has yielded early adapoids (Marcgodinotius, Asiadapis) and an omomyid (Vastanomys), described in 2005–09. They are among Asia’s oldest primates and link India’s fauna with Europe’s as India drifted towards Asia.

• Which family led to the anthropoids? Adapids were once favoured; most experts now prefer an Asian, tarsier-like source. Eosimias (China, about 45 million years ago) and Archicebus (China, about 55 million years ago, a tiny animal near the split between tarsiers and anthropoids) point to Asia as the cradle of the anthropoids, which later reached Africa.
• The “Ida” episode: in 2009 a 47-million-year-old skeleton from Messel, Germany (Darwinius) was promoted as a “missing link” to monkeys; it proved to be an adapid, closer to lemurs.
• Lemurs reach Madagascar: the island was already separated from Africa, so lemur ancestors probably rafted across on floating vegetation, then radiated without monkey competitors (Unit 1.5a §4.6).

1.4 Oligocene: the Fayum and the first anthropoids

Almost all early Old World anthropoids come from the Fayum depression, south-west of Cairo, Egypt, now desert but tropical swamp forest by the sea about 37–30 million years ago; Elwyn Simons led its excavation from the 1960s.

• Parapithecids (Apidium, Parapithecus): squirrel-sized leapers with three premolars (2.1.3.3), like New World monkeys, near the base of the anthropoids.
• Propliopithecids (Propliopithecus, Aegyptopithecus): with the catarrhine formula 2.1.2.3.
• Aegyptopithecus zeuxis (about 30–29 million years ago) is the best known. It weighed about 6–8 kg, had full postorbital closure, a small brain with an enlarged visual area and reduced smell bulbs, and the limbs of a slow, arboreal quadruped. Its canines varied greatly between individuals, implying sexual dimorphism and probably male competition for females.
• Significance: Aegyptopithecus is primitive enough to be close to the common ancestor of Old World monkeys and apes, bridging the Eocene prosimians and the Miocene apes.
• Other finds: Saadanius (about 29–28 million years ago, Saudi Arabia, 2010) is a catarrhine from just before the ape–monkey split. Rukwapithecus (about 25 million years ago, Tanzania, 2013), the oldest known ape, and its contemporary Nsungwepithecus, the oldest known Old World monkey, show the two lines had separated by the late Oligocene, as molecular dates predict.

How did monkeys reach South America? South America was an island continent, and the Atlantic was then much narrower (about 1,500–2,000 km). The oldest South American anthropoids, from Santa Rosa in the Peruvian Amazon, include Ucayalipithecus (2020), a parapithecid, a group otherwise known only from Afro-Arabia. Its ancestors probably rafted from Africa between about 35 and 32 million years ago, when sea levels dropped. The New World monkeys thus came from African stock, which explains why platyrrhines and catarrhines share anthropoid traits but have evolved separately ever since.

1.5 Miocene: the age of apes

If the Eocene was the age of prosimians, the Miocene (23–5.3 million years ago) was the age of apes: more than 500 individuals are known from Europe, Asia and Africa, many with no living descendants. Naming was long a muddle, with nearly 30 genera, two of them for the upper and lower jaws of one animal. In the 1960s Simons and Pilbeam “lumped” almost all into Dryopithecus and Ramapithecus; finds since the 1980s showed this went too far.

Early Miocene Africa (23–14 million years ago)

• Proconsul (about 20–17 million years ago, Rusinga Island and Koru, Kenya): named in 1933; the British archaeologist Mary Leakey found a nearly complete skull on Rusinga in 1948.
  • Ape-like: no tail, Y-5 molars, a fairly large brain.
  • Monkey-like: a flexible back and limbs of equal length, so it walked on top of branches.
  • It is a “dental ape“: an ape in its head and teeth, a monkey in its body. Most experts place it near or before the split of the small and large apes. The Rusinga species have since been renamed Ekembo.
• Morotopithecus (about 20 million years ago, Uganda): short, stiff lower back vertebrae and a mobile shoulder like living apes, showing an early upright-trunk (orthograde) body.
• Masripithecus (about 18–17 million years ago, northern Egypt, 2026): the first early-Miocene ape from North Africa (see Contemporary Developments).

Middle and late Miocene Europe (16–7 million years ago)

• Dryopithecus (first found at Saint-Gaudens, France, 1856, about 12–11 million years ago): thin-enamelled molars with the Y-5 pattern, a fruit-eater of forests. Its teeth gave the Y-5 its other name, the “Dryopithecus pattern“. It shows apes had spread into Europe, but it is not a direct human ancestor.
• Pierolapithecus (about 12 million years ago, Spain, 2004): broad chest and stiff lower back, the orthograde great-ape body plan.
• Danuvius (11.6 million years ago, Bavaria, Germany, 2019): complete limb bones show long arms for hanging and extended hips and knees, which its describers called “extended limb clambering“, a possible model for the common ancestor of great apes and humans. The claim of partial bipedalism is disputed.
• Ouranopithecus (about 9.5–8.7 million years ago, Greece): thick enamel; possibly near the African ape–human line.
• Oreopithecus (about 8–7 million years ago, Tuscany and Sardinia, Italy): an island ape with a short face and an unusual pelvis; treated in Unit 1.6a.

Middle and late Miocene Asia (16–7 million years ago)

• Sivapithecus (about 12.7–8.5 million years ago, Siwaliks of India and Pakistan; also Turkey): named by the British geologist Guy Pilgrim in 1910. It weighed about 30–70 kg, had thick-enamelled, flat-wearing molars and robust jaws. The 1980 face shows a concave profile, tall oval eye sockets set close together and projecting incisors, all orangutan features. Its arm bones, however, are unlike an orangutan’s, so it was probably not a suspensory specialist. This mismatch is still debated.
• Ramapithecus: now merged with Sivapithecus (above).
• Giant apes: Indopithecus (about 8.6 million years ago, Haritalyangar), a giant jaw first described as Gigantopithecus bilaspurensis (1969) by Simons and the Indian anthropologist S.R.K. Chopra of Panjab University; its Pleistocene relative Gigantopithecus is covered in §1.7.
• Kapi ramnagarensis (about 13 million years ago, Ramnagar, Jammu and Kashmir; Christopher Gilbert and colleagues, 2020): a single molar that is the oldest known gibbon fossil, about 5 million years older than the previous record. It shows that gibbons reached Asia at about the same time as the great apes.

Late Miocene Africa (10–6 million years ago): fossils are few, though this is where gorillas, chimpanzees and humans diverged; Chororapithecus (about 8 million years ago, Ethiopia, gorilla-like teeth) begins to fill the gap, and the first hominins, Sahelanthropus (Chad) and Orrorin (Kenya), appear at its end (Unit 1.6).

The table summarises the key fossils with their dates and significance.

Fossil	Site	Date (dating method)	Key traits	Significance
Purgatorius	Montana, USA	about 66 mya (ash beds, argon dating)	Fruit-eater’s molars; climbing ankle	Earliest primate-like mammal
Adapids / omomyids	N. America, Europe, Asia; Vastan, Gujarat	56–34 mya (marine microfossils at Vastan)	Nails, forward eyes, postorbital bar	First true primates
Aegyptopithecus	Fayum, Egypt	about 30–29 mya (argon dating of basalt; magnetic stratigraphy)	2.1.2.3; postorbital closure; small brain	Near the monkey–ape common ancestor
Proconsul	Rusinga, Kenya	20–17 mya (potassium–argon)	No tail; monkey-like body	Early “dental ape”
Dryopithecus	France, Spain, Hungary	12–9 mya (fossil fauna)	Thin enamel; Y-5 molars	Apes spread into Europe
Kapi	Ramnagar, J&K	13.8–12.5 mya (fossil fauna, magnetic stratigraphy)	Gibbon-like molar	Oldest gibbon
Sivapithecus	Siwaliks, India–Pakistan	12.7–8.5 mya (magnetic stratigraphy)	Thick enamel; orangutan-like face	Orangutan line; absorbed Ramapithecus
Danuvius	Bavaria, Germany	11.6 mya (magnetic stratigraphy)	Long arms, extended hips and knees	Model for the great ape–human ancestor
Gigantopithecus blacki	South China caves	2.3 mya – 295,000–215,000 years ago (luminescence, uranium-series, ESR)	Largest primate; huge flat molars	Quaternary giant ape; orangutan relative

Two patterns emerge: primates began as small arboreal fruit- and insect-eaters and diversified as the world cooled; and the record is poorest in Africa exactly when the human line began.

1.6 The Ramapithecus–Sivapithecus debate

The debate is examined in Paper II (Siwaliks and Narmada basin), but its logic belongs here. The table sets the two readings side by side.

Basis	Ramapithecus as a hominid (1960s–70s)	Ramapithecus as Sivapithecus (from about 1980)
Evidence	Fragmentary jaws; dental arcade reconstructed as parabolic (human-like)	More complete jaws and the 1980 face
Key traits	Small canines, thick enamel, short face	Same traits occur in female Sivapithecus; face orangutan-like
Human split	About 14 million years ago	About 6–7 million years ago
Supporting science	Fossil morphology alone	Fossils plus the molecular clock (Vincent Sarich and Allan Wilson, 1967)

The molecular clock (estimating the time since two species split from the number of differences in their proteins or DNA) had already suggested in 1967 that humans and African apes separated only about 5 million years ago. Palaeontologists at first rejected this; the 1980 face reconciled the two. The lesson is that thick enamel and small canines evolved more than once (convergence, Unit 1.4b §6.4) and cannot alone identify a hominin.

1.7 Quaternary fossil primates

The Quaternary (from 2.58 million years ago) adds the hominins of Unit 1.6 and several remarkable non-human primates.

• Gigantopithecus blacki (South China): the largest primate ever, known from four jaws and over a thousand teeth. It was discovered in 1935 by the German-Dutch palaeontologist Ralph von Koenigswald in Hong Kong drugstores, where its teeth were sold as “dragon teeth“. Estimates suggest up to about 3 m tall and 200–300 kg. Proteins from its enamel (2019) show it was a relative of the orangutan. It lived from about 2 million years ago until it became extinct between 295,000 and 215,000 years ago, when seasonal climates thinned its forests and it failed to adapt its diet (Contemporary Developments).
• Theropithecus oswaldi: a giant gelada baboon of Pleistocene Africa, nearly gorilla-sized, a grass-eater like the living gelada. Its bones in butchery sites suggest that early Homo hunted it.
• Giant lemurs of Madagascar: Archaeoindris (gorilla-sized), Megaladapis and Palaeopropithecus survived until the last 2,000 years, then died out after humans arrived, a Holocene extinction caused by hunting and forest clearance.

1.8 Fossil primates and human origins: an evaluation

• “India as the cradle of mankind”: Siwalik finds led Pilgrim (1915) and others to place the human line’s origin in India, and Ramapithecus revived the idea. But every early hominin is African, and the Siwalik apes lie on the orangutan line. India’s contribution is different:
  • Vastan holds some of the oldest primates of Asia;
  • the Siwaliks hold one of the world’s richest Miocene ape records (Sivapithecus, Indopithecus, Kapi);
  • the Narmada basin (Paper II) and Attirampakkam (Unit 1.4a) record later hominins and their tools.

India was a crossroads of primate evolution, not its cradle.

• “Dryopithecus holds the clue for human origin”: in the sense that dryopithecines show apes had the Y-5 teeth and the upright-trunk body from which both great apes and humans evolved, yes; as a direct ancestor, no.
• Gradualism versus punctuation: the American palaeontologist Philip Gingerich (1976) traced the Palaeocene–Eocene lineage of Plesiadapis through successive layers and found gradual size change, a primate example for gradualism; the long stasis of Homo erectus is the usual example for punctuated equilibrium (Unit 1.4b §5.3). Primate history fits both tempos at different times.

2. Living Major Primates

In the Hollongapar Gibbon Sanctuary in Jorhat, Assam, family groups of western hoolock gibbons sing loud duets at dawn to mark their territories. The hoolock is India’s only ape, living in the forests south and east of the Brahmaputra and in the north-eastern states. Its long arms, which almost touch the ground when it stands, and its swinging travel through the canopy are the features of all gibbons.

2.1 Lesser apes: gibbons and siamang (family Hylobatidae)

• Range: South-East Asia, southern China, north-east India, Bangladesh and Myanmar, south to Sumatra, Java and Borneo.
• Kinds: about 20 species in four genera: Hylobates, Hoolock (India’s western and eastern hoolocks), Nomascus and Symphalangus, the siamang of Sumatra and Malaysia, which is the largest and has a throat sac that amplifies its call.
• Body: about 5–8 kg (siamang up to 12), little sexual dimorphism, arms far longer than legs, hook-like hands with a short thumb, ischial callosities, brain about 100 cc.
• Locomotion: fast ricochetal brachiation (swinging with a flight phase); bipedal walking on branches and ground, arms raised.
• Society: pair-living, territories announced by duets, with some extra-pair mating.

2.2 Great apes

The table compares the four great apes; the text after it draws out what the contrasts mean.

Trait	Orangutan	Gorilla	Chimpanzee	Bonobo
Range	Borneo and Sumatra (Asia)	Equatorial central Africa	Equatorial Africa, Senegal to Tanzania	DR Congo, south of the Congo River
Species	Three: Bornean, Sumatran, Tapanuli (2017)	Two: western (lowland) and eastern (incl. mountain)	Pan troglodytes (four subspecies)	Pan paniscus
Male / female weight	about 80 / 38 kg	about 160 / 80 kg	about 45 / 35 kg	about 40 / 33 kg
Brain	about 400 cc	about 500 cc	about 390 cc	about 350 cc
Locomotion	Slow four-handed climbing (quadrumanous); fist-walking on the ground	Knuckle-walking; mainly on the ground	Knuckle-walking; climbs to feed	Knuckle-walking; more often bipedal
Social life	Largely solitary	One-male group led by a silverback	Male-bonded fission–fusion community	Female-led fission–fusion community
Diet	Fruit, especially figs	Leaves and stems (mountain); fruit (lowland)	Fruit; hunts monkeys	Fruit; little hunting
Status (IUCN)	All three Critically Endangered	Both Critically Endangered; mountain subspecies Endangered	Endangered	Endangered

• Asian versus African apes: the orangutan is the only great ape of Asia and the most arboreal; its separate lineage goes back about 13–16 million years, matching the Sivapithecus fossils (§1.5). The African apes are more terrestrial and knuckle-walk, and are closer to humans.
• Size and sex: gorillas and orangutans show the strongest sexual dimorphism (males about twice female weight); mature male gorillas grow silver back hair, hence “silverback”.
• Slow breeding: an orangutan mother gives birth only every 7–9 years, the longest interval of any mammal. Combined with habitat loss, this is why every great ape is endangered: a population cannot replace losses quickly.
• Chimpanzee and bonobo: the two Pan species are equally related to humans, yet they differ socially: chimpanzees are male-dominated, territorial and capable of lethal raids; bonobos are female-led and resolve tension through sex and play (Unit 1.5a §5.7). The difference is often linked to richer, more reliable food south of the Congo River, which lets bonobo females forage together and form alliances.
• Mountain gorillas (Virunga volcanoes and Bwindi): protection building on Fossey’s work raised them above 1,000, and they were moved from Critically Endangered to Endangered in 2018.

2.3 Living monkeys in brief

The major living monkeys are described with taxonomy in Unit 1.5a §3: in brief, the New World monkeys (marmosets, capuchins, spider and howler monkeys) are arboreal and often have prehensile tails; the Old World monkeys divide into the omnivorous, often terrestrial cercopithecines (macaques, baboons, guenons) and the leaf-eating colobines (langurs, colobus). India’s macaques and langurs belong to the Old World group.

3. Comparative Anatomy of Man and Apes

The frontispiece of Huxley’s 1863 book drew five skeletons side by side: gibbon, orangutan, chimpanzee, gorilla and man. The drawing makes the point at a glance: the same bones in the same arrangement, differing in proportion. Huxley concluded that “the structural differences which separate Man from the Gorilla and the Chimpanzee are not so great as those which separate the Gorilla from the lower apes.” Comparative anatomy measures those differences of proportion.

3.1 Similarities

Humans share with the apes all the hominoid features (Unit 1.5a §3.4):

• No tail; a broad, shallow chest; shoulder blades on the back; a mobile shoulder that lets the arm rotate above the head (we can hang from a bar and throw overarm because of this ancestry).
• Y-5 molars and the dental formula 2.1.2.3; an appendix; a large, folded brain; and the same slow life history with long infancy and single births.
• Physiology: similar blood groups (chimpanzees A and O, gorillas B), menstrual cycles and shared diseases.
• Genes: human and chimpanzee DNA differ at about 1.2 per cent of letters (about 4 per cent counting insertions and deletions). Humans have 46 chromosomes and great apes 48, because two ancestral ape chromosomes fused end to end to form human chromosome 2. Gene differences date the human–chimpanzee split to about 6–7 million years ago.

Which ape is closest? Each ape resembles humans in different features, which long confused the question:

• gorilla: the hands, feet and pelvis are most human-like among apes;
• chimpanzee: the skull and body pigmentation;
• orangutan: the high forehead and 12 pairs of ribs, as in humans (chimpanzees and gorillas have 13);
• gibbon: relatively long legs and frequent upright walking.

DNA settled it: Charles Sibley and Jon Ahlquist (American molecular biologists, 1984) and later genomes showed that chimpanzees and bonobos are closest, then gorillas, then orangutans. Single resemblances mislead; overall genetic distance does not.

3.2 The skull: gorilla and man

The gorilla skull shows the ape pattern in its most extreme form, so it is the standard comparison. The table lists the differences from the top of the head to the chin.

Feature	Gorilla	Man
Braincase and face	Face larger than braincase	Braincase larger than face
Brain size	about 500 cc	about 1,350 cc
Forehead	Absent; skull slopes back	High, vertical forehead
Brow ridge	Massive, continuous (supraorbital torus)	Small or absent
Crests	Sagittal crest on top (males) and nuchal crest at back, for huge chewing and neck muscles	No crests; smooth skull
Foramen magnum (opening for the spinal cord)	Towards the back; head hangs forward	Central, under the skull; head balanced
Mastoid process (bump behind the ear)	Small	Large (for neck muscles that balance the head)
Face	Prognathous (jaws project forward)	Orthognathous (face flat, under the brain)
Nose	Flat, broad nostrils, little bridge	Projecting nasal bones, prominent nose
Lower jaw	Massive; simian shelf (a bony ledge inside the chin); no chin	Light; chin present; no shelf
Dental arcade (shape of the tooth row)	U-shaped, parallel rows	Parabolic, diverging towards the back
Canines and diastema	Large, interlocking canines; diastema gap	Small canines, level with other teeth; no gap
Chewing	Mainly up and down (canines lock the jaws)	Side-to-side grinding (rotary)
Enamel	Thin	Thick

The skull differences have three causes, and knowing them makes the table easy to recall:

• Upright posture moved the foramen magnum forward and removed the need for heavy neck muscles and the nuchal crest.
• Brain expansion produced the high forehead and vaulted braincase.
• Smaller teeth and jaws, as tools and cooking took over cutting and softening food (Unit 1.4a §2), removed the sagittal crest, the brow ridge, the simian shelf and the interlocking canines; freed from canine locking, the jaw could grind from side to side.

3.3 The skeleton and body: chimpanzee and man

The postcranial skeleton (all of it below the head) differs mainly because of bipedalism, treated in §4. The table summarises.

Feature	Chimpanzee	Man
Spine	One C-shaped curve; 3–4 lumbar vertebrae	S-shaped, four curves; 5 lumbar vertebrae
Chest	Funnel-shaped (narrow at top)	Barrel-shaped
Pelvis	Tall, narrow blades facing backwards	Short, broad, bowl-shaped, blades curving round the sides
Limb proportions	Arms longer than legs (IMI about 105)	Legs longer than arms (IMI about 70)
Femur	Straight down from the hip; no angle at the knee	Slopes inward to the knee (valgus angle, about 8–11°)
Knee	Cannot lock straight; walks bent-hip, bent-knee	Locks straight; large joint surfaces
Foot	Grasping, opposable big toe; flat	Non-opposable big toe in line; arched; large heel
Hand	Long fingers, short thumb; knuckle-walking	Long, strong thumb; precision grip
Body hair	Dense coat	Sparse hair; many sweat glands

• Chimpanzee and man (1993 PYQ pattern): add to these the skull differences of §3.2 (milder in the chimpanzee than in the gorilla: smaller brow ridge, no sagittal crest in most) and the similarities of §3.1.
• Australopiths in between: fossil australopiths (Unit 1.6) sit between chimpanzee and human in canine size (smaller, non-projecting), enamel thickness (thicker), brain size (about 400–550 cc) and foramen magnum position (between rear and base), but already have a human-like knee and pelvis. Their teeth and hips modernised early, their brains late.

3.4 Evolutionary significance

• The differences are a map of hominization. They cluster in locomotion, brain and teeth, the three main trends of human evolution (Unit 1.4a §1.1). Each human trait can be traced to one of these changes.
• They appeared at different times. Fossils show the pelvis and knee changed first (by 4 million years ago), the teeth next, the brain last: mosaic evolution (Unit 1.4b §6.6). So no single feature makes a fossil human.
• Differences are mostly of degree. Huxley’s point stands: almost every human structure has an ape counterpart, enlarged, reduced or reshaped. What is new is the combination, plus the culture it made possible.
• The similarities have practical value. Shared anatomy and genes make apes models for medicine and for reconstructing early hominin behaviour, and they underline the ethical case for their conservation.

4. Skeletal Changes due to Erect Posture and Its Implications

Low back pain affected an estimated 619 million people in 2020 and is the world’s leading cause of disability (Global Burden of Disease study, 2023). It is not only modern: spinal arthritis appears in prehistoric hunter-gatherer skeletons. It is a price of standing on two legs with a skeleton remodelled from a tree-climbing ape. Unit 1.4a §2.1 summarised the changes; here they are taken region by region.

4.1 From head to foot

The table moves from the skull down. For each region it shows what changed and why the change helps an upright walker.

Region	Ape condition	Human condition	How it helps
Skull	Foramen magnum towards the back; strong neck muscles and crests	Foramen magnum central, under the skull; small neck muscles	The head balances on top of the spine with little muscle effort
Spine	One C-shaped curve	S-shaped: forward curves in the neck and lower back (lordosis), backward in the chest and sacrum	Places the head and trunk directly over the hips and feet; absorbs shocks like a spring
Vertebrae	Similar size down the column; 3–4 lumbar	Bodies grow larger downwards; 5 lumbar, wedge-shaped	Lower vertebrae carry more weight; wedging forms the lumbar curve
Sacrum	Long, narrow	Short and broad	Firm, wide link between spine and pelvis
Pelvis	Tall, narrow, blades at the back	Short, broad, bowl-shaped; blades (ilium) curve to the sides	Supports the abdominal organs from below; lowers the centre of gravity
Hip muscles	Gluteals mainly extend the hip	Gluteus medius and minimus at the side stop the pelvis tilting at each step; gluteus maximus enlarged	Keeps the body level on one leg while the other swings
Femur	Short, straight; no valgus angle	Long; slopes inward to the knee; larger head; strong linea aspera (muscle ridge at the back)	Brings the knees and feet under the body; long stride
Knee	Bent in walking; small joint	Locks fully straight; large condyles	Standing needs little muscle work
Foot	Grasping; opposable big toe; flat	Big toe in line; two arches (lengthwise and across); big heel bone	A rigid lever to push off and a spring to absorb impact
Upper limb	Long arms for climbing and knuckle-walking	Shorter arms; freed hand with long thumb	Hands free to carry and make tools

Two points tie the table together. The centre of gravity must sit over a tiny base, the two feet, so the spine curves, the femur angles inward and the knees lock to keep it there. And the foot changed most: from a grasping organ to a platform, with weight passing from the heel along the outer edge, across the ball, and off the big toe.

The curves form in every child: a newborn’s spine is one C-shaped curve, like an ape’s; the neck curve appears when the baby holds up its head (about 3–4 months) and the lower-back curve when it walks (about 12 months).

4.2 Why did bipedalism evolve?

Unit 1.4a §2.1 lists the main hypotheses; here they are set out with their evidence and weaknesses. They are not mutually exclusive.

Hypothesis	Proposed by	Core idea	Evaluation
Freeing the hands	Charles Darwin (The Descent of Man, 1871)	Hands freed for tools, weapons and carrying	Bipedalism appears about 4 million years before the first stone tools, so tools cannot be the first cause
Energy efficiency	Peter Rodman and Henry McHenry (1980)	Walking upright costs less energy than knuckle-walking when food patches are far apart	Strongly supported: human walking costs about 75 per cent less than chimpanzee walking (Michael Sockol and colleagues, 2007)
Keeping cool	Peter Wheeler (British physiologist, 1991)	Upright body exposes less skin to midday sun and catches cooler breezes	Mainly useful in open, hot country, but early hominins lived in woodland
Provisioning and pair-bonding	C. Owen Lovejoy (1981)	Males walked upright to carry food to mates and young	Assumes monogamy, which human societies do not universally show
Seed-eating and postural feeding	Clifford Jolly (1970); Kevin Hunt (1994)	Standing to pick seeds or fruit from low branches; bipedal walking grew from bipedal standing	Chimpanzees do stand to feed; fits a woodland setting
Arboreal origin	Susannah Thorpe and colleagues (2007)	Upright walking began in the trees, on flexible branches, as orangutans do	Supported by Danuvius (2019) and by the climbing features of early hominins
Endurance running	Dennis Bramble and Daniel Lieberman (2004)	Long-distance running for persistence hunting	Explains later features of Homo erectus, not the origin

The evidence now favours a two-stage view: bipedalism began as a way to move efficiently between scattered woodland foods, perhaps growing out of upright postures used in the trees and for feeding; carrying, tools, cooling and running became important later, as hominins moved into open country.

4.3 Gains and losses of erect posture

The new posture brought advantages and costs. The table balances them; the paragraph after it explains why the costs were never removed.

Gains	Losses
Hands freed for carrying food, infants and, later, tools	Slower sprinting than four-legged predators
Cheap walking over long distances	Loss of the grasping foot and of agile climbing
Less heat absorbed; easier cooling	Low back pain, slipped discs and stress fractures of the lower vertebrae
Wider view over tall grass; threat display	Knee and hip arthritis; flat feet and bunions
Endurance running (in Homo)	Varicose veins and haemorrhoids: blood must climb against gravity
A balanced head that could later carry a larger brain	Hernias, as abdominal organs press down on the groin
	Difficult childbirth (the obstetrical dilemma)

The costs persist because evolution modifies existing structures; it cannot start again. A quadruped’s spine is a horizontal bridge; ours is a vertical column whose lowest discs bear the whole upper body. The pelvis must be narrow for walking but wide for a large-headed baby, so the infant must rotate through the birth canal: Sherwood Washburn’s obstetrical dilemma (1960). The American anthropologist Holly Dunsworth and colleagues (2012) argued that birth timing is set rather by the mother’s energy limit (the “EGG” hypothesis). Either way, human birth became slow and risky, and assisted birth a cultural universal.

4.4 Implications for reading fossils and for human evolution

• Reading bipedalism from bones: because the changes run from skull to foot, a single bone can show bipedalism.
  • The forward foramen magnum of the Taung child (1925) was the first clue that australopiths walked upright.
  • The valgus knee of Australopithecus afarensis (about 3.2 million years ago) shows the femur already angled inward.
  • The Laetoli footprints (about 3.66 million years ago) show an arched foot with the big toe in line.
  • A femur of Sahelanthropus (about 7 million years ago), described in 2022, was read as bipedal, but its forearm bones show climbing, and many experts dispute the femur.
• Bipedalism came first: “feet before brains“, which overturned the old belief, behind the Piltdown forgery, that the brain led (Unit 1.4a).
• Early bipeds were mixed. Ardipithecus (about 4.4 million years ago) walked upright on the ground but still had a grasping big toe for climbing; australopiths had human-like hips with curved fingers. Habitual bipedalism with a fully human foot came only with Homo erectus (Unit 1.6b).
• Bipedalism made culture possible: free hands could carry and make tools, and the obstetrical dilemma produced helpless infants and cooperative care, the setting in which culture is learned.

Key Terms

Term	Meaning in simple words
Tertiary / Quaternary	The periods from 66 to 2.58 million years ago, and from 2.58 million years ago to now
Adapids / omomyids	Eocene lemur-like / tarsier-like primates
Dental ape	A fossil with ape teeth but a monkey-like body, such as Proconsul
Siwaliks	Himalayan foothill deposits rich in Miocene ape fossils
Molecular clock	Dating a split between species from the number of genetic differences
Brachiation	Swinging hand over hand beneath branches
Sagittal crest	A bony ridge on top of the skull for large chewing muscles
Simian shelf	A bony ledge inside the front of an ape’s lower jaw
Diastema	A gap in the tooth row that receives the opposite canine
Foramen magnum	The opening in the skull base through which the spinal cord passes
Lumbar lordosis	The forward curve of the lower back
Valgus (bicondylar) angle	The inward slope of the femur to the knee
Orthograde	Holding the trunk upright
Obstetrical dilemma	The conflict between a narrow walking pelvis and a large-headed baby

Contemporary Developments

• An early ape from Egypt (March 2026): jaws about 17–18 million years old from Wadi Moghra were named Masripithecus, the first early-Miocene ape of North Africa; its describers place it closer to living apes than any East African ape of its age and suggest the living apes arose in north-east Afro-Arabia (Shorouq Al-Ashqar and colleagues, Science), a claim others call premature.
• Complete ape genomes (April 2025): the chimpanzee, bonobo, gorilla, two orangutan and siamang genomes were sequenced end to end (telomere to telomere), allowing exact comparison with the human genome (DongAhn Yoo and colleagues, Nature).
• Why the giant ape died out (January 2024): Gigantopithecus blacki became extinct 295,000–215,000 years ago, as seasonal climates opened its forests and it failed to change its diet (Yingqi Zhang and colleagues, Nature).
• Bipedalism at 7 million years? (August 2022): a femur and two forearm bones of Sahelanthropus from Chad were described as showing habitual bipedalism combined with climbing (Guillaume Daver and colleagues, Nature); the reading of the femur remains disputed.
• India’s oldest gibbon (2020): a molar from Ramnagar, Jammu and Kashmir, about 13 million years old, was named Kapi ramnagarensis, the earliest known gibbon, extending the fossil record of the lesser apes by about 5 million years.

Diagrams to Master

1. Topic map: fossil primates, living apes, man–ape anatomy and erect posture.
2. Fossil timeline: epochs with start dates (66, 56, 33.9, 23, 5.3, 2.58 million years ago) and key fossils; mark Vastan, the Siwaliks and the Fayum.
3. Living apes: gibbon, orangutan, gorilla, chimpanzee, bonobo by range, weight, brain, locomotion and society.
4. Skull comparison: gorilla and human in profile; label sagittal crest, brow ridge, foramen magnum, prognathism, canines, chin, and the U-shaped versus parabolic tooth row.
5. Erect posture: ape and human skeletons with foramen magnum, S-curve, bowl pelvis, valgus femur, locked knee, arched foot and the line of gravity through the feet.

Revision Grid

Scholar	Work (year)	Key idea
Huxley	Evidence as to Man’s Place in Nature (1863)	Man and great apes differ less than great apes and monkeys
Darwin	The Descent of Man (1871)	Bipedalism freed the hands
Pilgrim	1910	Named Sivapithecus
Lewis	1934	Named Ramapithecus (Haritalyangar)
Mary Leakey	1948	Proconsul skull, Rusinga
Washburn	1960	Obstetrical dilemma
Simons and Pilbeam	1965	Ramapithecus as a hominid; lumping of Miocene apes
Sarich and Wilson	1967	Molecular clock: human–ape split about 5 million years ago
Simons and Chopra	1969	Gigantopithecus bilaspurensis (Indopithecus), Haritalyangar
Rodman and McHenry	1980	Energy efficiency of bipedalism
Lovejoy	1981	Provisioning hypothesis
Pilbeam	1982	Sivapithecus face linked to orangutan
Sibley and Ahlquist	1984	Chimpanzees closest to humans by DNA
Wheeler	1991	Thermoregulation hypothesis
Bramble and Lieberman	2004	Endurance running
Sockol and colleagues	2007	Human walking about 75 per cent cheaper
Dunsworth and colleagues	2012	Energetic limit on gestation (EGG)